Conventionally, as a method for utilization of a semiconductor material functioning as a photocatalyst, it is known to decompose water to produce hydrogen or to generate electrical energy by irradiating a semiconductor material with light (for example, PTL 1).
PTL 1 discloses a photo-assisted water electrolysis apparatus having a function of converting light energy obtained from solar light to hydrogen energy. The photo-assisted water electrolysis apparatus is composed of a plurality of laminated photo-assisted water electrolysis cells.
Each photo-assisted water electrolysis cell has a box-like casing whose peripheral portion is surrounded with an outer wall made of a transparent glass or synthetic resin plate, and is arranged in a state of being inclined at a given angle from the horizontal state. An electrolyte is accommodated in a lower portion of the photo-assisted water electrolysis cell, and a separation wall which divides the photo-assisted water electrolysis cell into two spaces is provided in the middle of the thickness direction of the cell. The separation wall is formed by integrally joining a gas separation membrane arranged on an upper side with a photo-assisted water electrolysis electrode/membrane assembly arranged on a lower side, and plays a role of separating the produced hydrogen from the produced oxygen.
In the photo-assisted water electrolysis electrode/membrane assembly, a photocatalyst electrode and a platinum counter electrode are respectively formed on both surfaces of a Nafion membrane which is an ionic conductive membrane arranged in the middle of the thickness direction. In the photo-assisted water electrolysis electrode/membrane assembly, irradiation of solar light causes the photo-assisted water electrolysis, and oxygen is produced from the photocatalyst electrode and hydrogen is produced from the platinum counter electrode. Further, the lower end of the separation wall is provided with a rectangular through hole, and the electrolyte can be circulated within the photo-assisted water electrolysis cell through the through hole.
Further, in an outer wall of the photo-assisted water electrolysis cell, a rectangular circulation hole in planar view is formed, and a movable wall which makes the opening area of the circulation hole freely variable is provided. The movable wall is configured slidably along a height direction (longitudinal direction) of an outer wall, and the opening area of an opening is decreased when the movable wall moves upward, and the opening area of an opening is increased when the movable wall moves downward. Herein, an upper end of the photo-assisted water electrolysis electrode/membrane assembly is arranged at substantially the same height as an upper end of the movable wall.
Further, a foot of a perpendicular drawn from the upper end of the photo-assisted water electrolysis electrode/member assembly toward the outer wall of the photo-assisted water electrolysis cell agrees with a position of a lower end of the circulation hole. Thus, a liquid height of the electrolyte in the photo-assisted water electrolysis cell substantially agrees with a height of the upper end of the photo-assisted water electrolysis electrode/membrane assembly and that of the upper end of the movable wall. A circulation hole is disposed in the outer wall of the photo-assisted water electrolysis cell and configured so as to enable circulation of the electrolyte through the circulation hole between neighboring photo-assisted water electrolysis cells.
Therefore, from the viewpoint of a flow of the electrolyte, in each photo-assisted water electrolysis cell, the electrolyte having flown through a circulation hole on an upstream side passes through the through hole and is flown out from a circulation hole on a downstream side. When the photo-assisted water electrolysis cells are connected in series in relation to the flow of the electrolyte, this enables supply and discharge of the electrolyte in all photo-assisted water electrolysis cells.
However, in the case of the photo-assisted water electrolysis apparatus, the space of each photo-assisted water electrolysis cell on a photocatalyst electrode side is communicated with a space of a photo-assisted water electrolysis cell on a platinum counter electrode side which is upwardly adjacent to this photo-assisted water electrolysis cell through a circulation hole. Similarly, the space of each photo-assisted water electrolysis cell on a photocatalyst electrode side is communicated with a space of a photo-assisted water electrolysis cell on a photocatalyst electrode side which is downwardly adjacent to this photo-assisted water electrolysis cell through a circulation hole.
Therefore, hydrogen and oxygen produced in each photo-assisted water electrolysis cell are easily mixed with each other through a circulation hole. Further, a portion of oxygen bubbles produced at the surface of the photocatalyst electrode is swept away beyond the through hole by the electrolyte, enters the space on the platinum counter electrode side and is mixed with hydrogen bubbles produced at the surface of the platinum counter electrode.
That is, in the photo-assisted water electrolysis apparatus, it is not possible to collect hydrogen and oxygen separately because of the structure even though a location of hydrogen production is different from that of oxygen production.